Cardiac arrhythmias, commonly known as irregular heart beats or racing hearts, are the result of various physical defects in the heart itself. One such defect is an extraneous strand of muscle fiber in the heart that provides an abnormal short-circuit pathway for electric impulses traveling through the heart tissue. This accessory pathway often causes the electric impulses that normally travel from the upper to the lower chamber of the heart to be fed back to the upper chamber, causing the heart to beat irregularly and therefore inefficiently pump blood.
Another common type of cardiac arrhythmia is ventricular tachycardia (VT), which may be a complication resulting from a heart attack or from a temporary reduction of blood supply to an area of heart muscle. VT is often caused by a tiny lesion, typically on the order of one to two millimeter, that is located close to the inner surface of the heart chamber. That lesion is often referred to as an “active site”, because it does not fire in sequence with the rest of the heart muscle. VT causes the heart's normal rhythmic contraction to be altered, thereby affecting heart function. A typical symptom is rapid, inefficient heart beats.
Minimally invasive techniques have been developed which are used to locate cardiac regions responsible for the cardiac arrhythmia, and also to disable the short-circuit function of these areas. According to these techniques, electrical energy shocks are applied to a portion of the heart tissue to ablate that tissue and produce scars which interrupt the reentrant conduction pathways. The regions to be ablated are usually first determined by endocardial mapping techniques. Mapping typically involves the percutaneous introduction of a diagnostic catheter having one or more electrodes into the patient, passing the diagnostic catheter through a blood vessel. (e.g. the femoral vein or aorta) and into an endocardial site (e.g., the atrium or ventricle of the heart), and inducing a tachycardia so that a continuous, simultaneous recording can be made with a multichannel recorder at each of several different endocardial positions. When a tachycardia focus is located, as indicated in the electrocardiogram recording, it is marked by means of a fluoroscopic image so that cardiac arrhythmias at the located site can be ablated. An ablation catheter with one or more electrodes can then provide electrical energy to the tissue adjacent the electrode to create a lesion in the tissue. One or more suitably positioned lesions will create a region of necrotic tissue to disable the malfunction caused by the tachycardia focus.
Ablation is carried out by applying energy to the catheter electrodes once the electrodes are in contact with the cardiac tissue. The energy can be, for example, RF, DC, ultrasound, microwave, or laser radiation. When RF energy is delivered between the distal tip of a standard electrode catheter and a backplate, there is a localized RF heating effect. This creates a well-defined, discrete lesion slightly larger than the tip electrode (i.e., the “damage range” for the electrode), and also causes the temperature of the tissue in contact with the electrode to rise.
Often, to overcome cardiac arrhythmias such as atrial flutter and atrial fibrillation, it is necessary to create a long, continuous lesion (i.e., a linear lesion). However, in order to maintain sufficient flexibility in the catheter shaft so that it may bend and assume requisite configurations to establish proper tissue contact, the conventional ring electrodes mounted on ablation catheters must be kept relatively short. Thus, to form a long, continuous lesion, clinicians have been forced to perform what is commonly referred to as a “drag” method, in which an ablation electrode is dragged along the patient's tissue while ablation energy is delivered to the electrode to scar the adjacent tissue to create a lesion. Such methods suffer from a number of disadvantages. For example, once the portion of the catheter shaft carrying the ablation electrode is making good tissue contact, it is undesirable to move the catheter shaft, because of the risk of losing the tissue contact. In addition, if the electrode is dragged too quickly, the tissue will not be sufficiently heated to scar.
Others have attempted to overcome this problem by incorporating a relatively long, cylindrical electrode mounted over the catheter shaft. The relatively long electrode can create longer lesions without requiring that the electrode (and thus the catheter shaft) be moved. However, using long electrodes also has significant drawbacks, one being that an elongated electrode detracts from the flexibility of the catheter, such that the catheter may not be able to assume a desired curve due to the straightening effects of the elongated electrode(s).
Accordingly, it will be apparent that there continues to be a need for a device for performing ablations which facilitates the creation of linear lesions. In addition, there exists the need for a device which does not require the surgeon to physically drag the catheter shaft to create a linear lesion. The instant invention addresses these needs.